In recent years, the use of readers which are adapted to optically read labels containing alpha-numeric or encoded information, such as bar codes and two dimensional symbologies, has become widespread. Traditional bar code readers, such as the familiar scanning devices used in many grocery stores, operate by scanning with a laser beam the surface of an object on which the bar code is formed. The image that is reflected from the code surface is received by a light sensing element and converted into a binary signal corresponding to the information contained in the label or bar code.
The growth in the use of code readers has been particularly strong in the parcel delivery industry, where information is printed on labels affixed to merchandise, packages, letters, moving objects and other items being shipped or transported. Modem optically-readable labels, such as the one described in U.S. Pat. No. 4,874,936 to Chandler et al., and which is incorporated herein by reference, comprise two-dimensional symbologies to provide a much higher information-density than conventional bar codes. Typically the encoded information on these labels includes information regarding origin, flight number, destination, name, price, part number, etc. Two-dimensional encoded labels are also widely used for automated routing and sorting of mail, parcels, baggage, etc.
In order to read the alpha-numeric and/or encoded labels, the parcel delivery industry commonly uses "over the belt" camera systems, in which overhead cameras create images of bar code labels or the like on packages traveling on conveyor belts below the camera. Typically these camera systems utilize charge-coupled device ("CCD") cameras to produce such images. Modem over the belt camera systems have conveyor belt widths of one and a half feet to five feet to accommodate packages of varying sizes and shapes, and belt speeds of up to five hundred feet per minute.
In order to read and create images of labels containing alpha-numeric or encoded symbologies, the over the belt camera system must be able to focus on the label as it rapidly passes on the conveyor belt. Packages traveling on the conveyor belt will be of varying heights, so the distance between the label and the: camera sensing element may vary significantly. Often the height of the packages will vary to such an extent that the label falls outside of the camera's depth of field, thereby preventing the optical decoding system from focusing on the label.
A camera in a typical "over the belt" reader system operates by taking "slices" of the image of the label as the object passes below on the conveyor belt. It will be appreciated that the width of the image is determined by the field of view across the belt, while the length of the image is determined by the velocity of the belt. The "aspect ratio" is defined as the ratio of the height of the image to the width of the image. In order to prevent the image from becoming distorted, it is important that the aspect ratio be kept constant. However, if the object-to-detector distance varies while the belt speed remains constant, only the width of the image changes, and the image becomes distorted.
The difficulty in maintaining a focused image has led to the development of various autofocusing techniques. One conventional method of autofocusing is disclosed in U.S. Pat. No. 4,877,949 to Danielson et al. In this method, the focusing lens of the camera is moved by a motor to maintain the image and focus. However, changing the front or back focal length to refocus as the distance between the label and the camera changes has the drawback that the magnification changes when the refocusing occurs. That is, the closer the encoded label is to the sensing element of the camera, the larger the image size appears. In addition, moving the focusing lens does not compensate for the change in aspect ratio, permitting distortion of the image. This approach has proven unsuitable for use in situations where the distance between the object and the sensing element varies greatly.
U.S. Pat. No. 5,308,966 to Danielson et al. discloses an optical system comprising a plurality of mirrors arranged at different distances to provide multiple optical paths between a hand-held bar code reader and a label. This system, however, requires a separate image sensor to be placed in each of the multiple optical paths.
Another conventional method of autofocusing relies on physically moving the position of the camera to compensate for varying object heights. This method is limited by the need to overcome the inertia of the camera in its linear motion, and permits changes in the aspect ratio.
According to yet another prior method directed to a similar problem, the optical path length between a light source and an object is varied in order to compensate for varying reading distances. In this technique, as described in U.S. Pat. No. 5,216,230 to Nakazawa, a reflecting mirror is moved into and out of the optical path between the light source and the object in order to provide two different optical path lengths. Additional reflecting mirrors may be arranged to be moved into and out of the optical path in order to provide additional optical path lengths. However, because this method requires multiple mechanical arrangements for driving the reflecting mirrors into and out of the optical path, this technique is unsuitable for providing a multiplicity of optical path lengths.
U.S. Pat. No. 5,185,822 to Miura describes an automatic sorting system for objects on a conveyor. In order to focus a camera on objects of differing sizes, the system translates a pair of reflecting mirrors along the optical path between a light source and the object. It appears this system would require parts made with a very high degree of precision to assure that the mirrors would stay in alignment as they were being moved.
Therefore, there is a need for an optical system that ensures that the image of a label is not distorted by maintaining a substantially constant aspect ratio as the distance between the camera and the label changes.
There is also a need for an autofocusing system in which the size of the image of a label remains substantially constant despite changes in the distance between the camera creating the image and the label.
There is also a need for an optical system that equalizes the optical path length between a camera and an object without changing the focusing optics or the camera position.
There is also a need for an optical system that provides a plurality of optical path lengths between a camera and an object and that does not require the use of multiple image sensors.
Furthermore, there is a need for an optical path equalizer that avoids the need for multiple camera sensors or multiple lens systems.